It is known that the behavior of a tire on the road, especially car tires, mainly depends on the composition of the tread blend. Numerous studies are consequently being effected to vary the compositions of the elastomeric polymer, fillers and other additives contained in the tread blend, to find increasingly improved compositions.
It should also be taken into consideration that an improvement in one property of the tire, can lead to a deterioration in other characteristics. It is well known, for example, that an improvement in the performances on a wet surface is normally linked to a deterioration in the rolling resistance.
Normal blends comprising carbon black as reinforcing material have proved to be unsuitable for obtaining a balance between the rolling resistance and wet skid properties.
Better results have been obtained using silica as the prevailing reinforcing material.
Unlike carbon black, however, silica has the disadvantage of a having a poor affinity for the elastomeric material, with consequent problems of processability of the blend due to the low dispersibility of silica in the elastomeric material. A first problem to be solved when using silica is therefore to have a good dispersion of the silica in the rubber. This problem can be solved by using a coupling agent based on silane, modifying the rubber and increasing the affinity of silica for rubber.
EP-A-775725 describes a raw elastomeric composition comprising (i) at least 30 phr (parts by weight referring to 100 parts of the total rubber) of a styrene/partially hydrogenated butadiene copolymer; (ii) silica; (iii) an organosilane as coupling agent. Once vulcanized, this blend produces tire treads with good performance. In the above document, however, the bonds between the macromolecules are not C—C bonds but C-heteroatom (Si, O, Sn) bonds. The hydrogenation conditions used therefore cause a partial degradation of the molecules with a consequent decrease in the Mooney viscosity. This is probably due to the interaction of the C-heteroatom bonds with the alkylating agents (for example Al-alkyls, Mg-alkyls and Li-alkyls) which are used for activating the metal of the transition series normally used as a hydrogenation catalyst. Furthermore, the use of coupling agents capable of reacting with the active terminals of the polymeric chains (consisting of lithium dienyls or lithium styryls) causes the formation of radial structures of the star type, wherein the length of the branches is the same if the parent polymer has been synthesized in a batch reactor or different when the parent polymer has been synthesized in a continuous reactor; in this second case, the molecules with a lower molecular weight are preferably coupled, as the relative molar concentrations are more abundant. The result is an insignificant branching from a rheological point of view, in particular to achieve important results in terms of efficiency in the dispersion of the fillers during the preparation phase of the blend.
It has been found that, by using a partially hydrogenated vinylarene conjugated diene copolymer, having a branched structure, in which the molecules are bound by carbon-carbon bonds and wherein the branching is mainly oriented on the high or very high molecular weight chains, treads can be obtained with enhanced properties with respect to the treads obtained using the linear copolymers described in EP-A-775725.